PROJECT SUMMARY Calmodulin (CaM) regulation of CaV1-2 channels?termed calmodulationhas proved enormously rich, both biologically, and as a general modulatory prototype with discriminating Ca2+ decoding capabilities. Mechanistically, a single Ca2+-free CaM (apoCaM) preassociates to the intracellular carboxy terminus of channels. Ca2+-binding to this resident CaM induces as-yet- unclear conformational changes that facilitate (CDF) or inactivate (CDI) channel opening, casting calmodulation as a positive or negative feedback control system for Ca2+. Intriguingly, Ca2+- binding to the C- and N-terminal lobes of CaM can each induce distinct forms of channel regulation, echoing earlier findings of CaM ?functional bipartition? in Paramecium. Beyond reach, however, is knowledge of subsystem interaction and dynamics; these deficits critically limit mechanistic advance and articulation of the biological impact of calmodulation. Moreover, the field is at a standstill regarding the structural underpinnings of calmodulation, owing in part to the difficulty that we and others have experienced in obtaining atomic structures of modules much beyond the small IQ domain. This structural indeterminacy fundamentally obscures the linkage of promising functional mechanisms to molecular reality. This project integrates multiple state-of- the-art strategies to transcend these obstacles according to three specific aims. 1) To clarify functional mechanisms of calmodulation via dynamic control of Ca2+ inputs to channels. 2) To identify the molecular states underlying CaM regulation of CaV channels. 3) To harvest mechanistic advances for biological insight and future therapeutics. In all, this project will sharply accelerate an already fruitful era of discovery, promising valuable advances regarding the function, structure, and biological aspects of CaV calmodulation.